This invention relates to a fluidized bed heat exchanger and a method of operating same in which heat is transferred from hot particulate material which flows through a plurality of chambers.
Various types of reactors, or heat exchangers, such as steam generators, or the like, utilize a fluidized bed as the primary medium of heat transfer. In these arrangements, air is passed through a bed of particulate material to fluidize the bed.
The most typical fluidized bed system is commonly referred to as a "bubbling" fluidized bed in which a bed of particulate materials is supported by an air distribution plate, to which air is introduced through a plurality of openings in the plate, causing the material to expand and to take on a suspended, or fluidized, state. The gas velocity is typically two to three times that needed to develop a pressure drop at which will support the bed weight (e.g., minimum fluidization velocity), causing the formation of bubbles that rise up through the bed and give it the appearance of a boiling liquid. The bed exhibits a well-defined upper surface, and the entrainment of particles in the gas leaving the bed is quite low, such that collection and recycle of these particles is not always necessary. The heat and mass transfer properties of the two-phase mixture are high, being typical of a liquid.
In these type arrangements, heat exchange surfaces are often immersed in the bed of fluidized particulate material to remove heat from the material and utilize the heat for other purposes. When such an immersed heat exchanger is used, it is often desirable to be able to control the rate at which the heat is extracted. This is usually done by varying the fluidized bed height and therefore the quantity of surface that is immersed.
However, situations exist in which a sufficient degree of freedom in choosing bed height is not available, such as for example, when a minimum fluidized bed solids depth or pressure is required for reasons unrelated to heat transfer. In this case, the heat transfer may be controlled by diverting a portion of the particulate material so it does not contact and become cooled by the heat exchanger. When the correct portions of cooled and uncooled material are subsequently recombined, the desired final material temperature may be obtained. For example, different type valves, such as "plug valves" and "L valves," have been used to bleed a portion of the solids passing through the bed and/or to directly control the flow rates from both a heat exchanger bed and a material-pressure control bed. However, these type arrangements require the use of moving parts within the solids system and/or need solids flow conduits with associated aeration equipment.